InP-based 1.3 .mu.m wavelength lasers which operate at high temperatures and are intended for low threshold and/or low bias current operation are believed to play an important role as transmitters in future access networks, see S. Yamashita et al. "Low-threshold (3.2 mA per element) 1.3 .mu.m InGaAsP MQW laser array on a p-type substrate", IEEE Photonics Technology Letters, vol. 4, No. 9, pp. 954-957, 1992 and H. Nobuhara et al. "1.3 .mu.m wavelength, low-threshold strained quantum well laser on p-type substrate", Electronic Letters, vol. 30, No. 16, pp. 1292-1293, 1994.
Due to the expected high temperatures it is crucial to improve their poor temperature dependence, which is believed to be partly caused by poor carrier confinement or carrier leakage in the growth, transverse direction, see P. A. Andrekson et al. "Effect of thermionic electron emission from the active layer on the internal quantum efficiency of InGaAsP lasers operating at 1.3 .mu.m", IEEE Journal of Quantum Electron Electronics, vol. 30, No. 2, pp. 219-221, 1994 and H. Ishikawa et al. "Analysis of temperature dependent optical gain of strained quantum well taking account of carriers in the SCH layer", IEEE Photonics Technology Letters, vol. 6, No. 3, pp. 344-347, 1994.
Thus a material is desired which has a conduction band higher than InP, i.e. a barrier, which can be epitaxially grown on InP. Materials which have been proved to have this property may contain aluminium what has also has been demonstrated by the use of alloys containing aluminium (Al) as barrier materials in lasers, see C. E. Zah et. al. "High-performance uncooled 1.3 .mu.m Al.sub.x Ga.sub.y In.sub.1-x-y As/InP strained-layer quantum-well lasers for subscriber loop applications", IEEE Journal of Quantum Electronics, Vol. 30, No. 2, pp. 511-523, 1994 and U.S. Pat. No. 5,381,434, which are incorporated herein by reference.
However, the threshold currents at room temperature of lasers having layers (barriers) containing Al for confinement of charge carriers and photons are not as low as the lowest ones reported for lasers not containing Al. The resulting operating currents at higher temperatures show no improvement compared to the best reported Al free lasers: between 30 and 45 mA for 6 mW output power @ 85.degree. C. for facet-coated Fabry-Perot lasers, see e.g. Table 1 in H. P. Mayer et al.: "Low cost high performance lasers for FITL/FTTH", The 21st European Conference on Optical Communications, ECOC '95, Brussels, September 1995, Proceedings Volume 2, Regular Papers & Invited Papers, pp. 529-536, 1995. The reliability or life time of lasers containing Al compared to lasers not containing Al is a critical issue, since alloys containing Al may react, in the manufacture of such lasers, by oxidation when exposed outside the chamber for epitaxial growth during etching and regrowth processing.
The experimental results obtained by C. E. Zah et al.: "High-performance uncooled 1.3 .mu.m Al.sub.x Ga.sub.y In.sub.1-x-y As/InP strained-layer quantum-well lasers for subscriber loop application", IEEE Journal of Quantum Electronics, Vol. 30, No. 2, pp. 511-523, 1994 and as disclosed in U.S. Pat. No. 5,381,434 showed, however, no increased problem with reliability. This can be explained, since for their measured results, they used a ridgetype laser structure which does not require exposure of the Al containing layers, contrary to a conventional buried heterostructure. However, a buried heterostructure has the advantage of providing a precise control of the width of the current confinement, resulting in a lower threshold current at room temperature.